Field of the Invention
The present invention relates to a focus detection apparatus, a focus detection method, and an image capturing apparatus, and more specifically to a focus detection apparatus, a focus detection method, and an image capturing apparatus that perform autofocus control based on an on-imaging plane phase difference detection method.
Description of the Related Art
Image capturing apparatuses, such as cameras, utilize autofocus (AF) methods, one example of which is a phase difference detection method (hereinafter, “phase difference AF”). With phase difference AF, a light beam that has passed through an exit pupil of an imaging lens is divided into two, and a pair of focus detection sensors receive the two divided light beams, respectively. Then, a shift amount of signals output from the pair of focus detection sensors, that is, a relative positional shift amount in the direction of light beam division (hereinafter, “image shift amount”), is detected. A focus lens is controlled by obtaining a shift amount in a focusing direction of the imaging lens (hereinafter, “defocus amount”) from the image shift amount thus obtained.
On-imaging plane phase difference AF has been proposed that provides a phase difference detection function to an image sensor, thereby rendering dedicated focus detection sensors unnecessary, and achieving phase difference AF at high speed. On-imaging plane phase difference AF provides a pupil division function by dividing a photoelectric conversion unit of a pixel of the image sensor into two parts, performs focus detection by individually processing the outputs from the two divided parts, and uses a sum of the outputs from the two divided parts as an image signal.
Japanese Patent Laid-Open No. 2011-247938 discloses a configuration related to a method of adding focus detection signals in on-imaging plane phase difference AF; this configuration detects whether the position of a subject has changed, and changes the number of frames of focus detection signals to be added based on the detection result. Furthermore, Japanese Patent Laid-Open No. 2014-32214 discloses a method of setting the number of luminances to be added, as well as the number of correlation curves to be added after correlation calculation, to suppress noise of focus detection signals.
Although the configurations of Japanese Patent Laid-Open No. 2011-247938 and No. 2014-32214 aim to increase the accuracy of focus detection by suppressing noise of focus detection signals through addition of the focus detection signals in a time direction and a space direction, they do not take a calculation period into consideration. Therefore, they face the problem of a long calculation period in performing high-accuracy focus detection. Especially, when subject tracking or prediction is performed, a reduction in a calculation period is demanded due to the necessity of sampling in a time direction.